Antennas 100 may be hidden behind a radome 110, see FIG. 1, particularly if they are being used in an application where they could be exposed to the environment. The radome protects the antenna from both the natural environment such as rain and snow, and the man-made environment such as jamming signals. Often, the radome is made so that it transmits electromagnetic energy within a narrow band centered around the operating frequency of the antenna, so as to deflect or reflect jamming signals at other frequencies. This is done using a frequency selective surface (FSS), having a grid or lattice of metal patterns or holes in a metal sheet. The design and construction of FSSs is well known to those skilled in the art of radome design and electromagnetic material design.
Two surfaces are commonly used in FSS design, the “Jerusalem cross” structure 200, shown in FIG. 2a, and its “Inverse structure” 300, shown in FIG. 3a. A unit cell equivalent circuit 201 of the Jerusalem cross 200, FSS can be viewed as a lattice of capacitors 210 and inductors 220 in series, shown in FIG. 2b. The capacitors 210 and inductors 220 are oriented in two orthogonal directions so that the surface can affect both polarizations. Near the LC resonance frequency, the series LC circuit has low impedance, and shorts out the incoming electromagnetic wave, thereby deflecting it off the surface. At other frequencies, the LC circuit is primarily transmitting, although it does provide a phase shift for frequencies near the stop band, shown in FIG. 2c. 
The Inverse structure 300, shown in FIG. 3a, has opposite characteristics. A unit cell equivalent circuit 301 of the Inverse structure 300, FSS can be viewed as a lattice of capacitors 310 and inductors 320 in parallel, shown in FIG. 3b. It is transmissive near LC resonance frequency and reflective at other frequencies, shown in FIG. 3c. 
The radome typically transmits RF energy through the radome only at the operating frequency of the antenna, and reflects or deflects at other frequencies. In some applications, it may be desirable to tune the radome, particularly when a tunable antenna is used inside the radome. It may also be desirable to set the radome to an entirely opaque (off) state, so that it is deflective or reflective over a broad range of frequencies. It may also be desirable to program the radome so that different regions have different properties, either transmitting within a frequency band, or opaque as desired. To achieve these requirements the FSS needs to be tunable.
Throughout the years, different techniques have been implemented to achieve the tuning of the FSS. The tuning has been achieved by: varying the resistance, see Chambers, B., Ford, K. L., “Tunable radar absorbers using frequency selective surfaces”, Antennas and Propagation, 2001. Eleventh International Conference on (IEEE Conf. Publ. No. 480), vol. 2, pp. 593-597, 2001; pumping liquids that act as dielectric loading, see Lima, A. C. deC., Parker, E. A., Langley, R. J., “Tunable frequency selective surface using liquid substrates”, Electronics Letters, vol. 30, issue 4, pp. 281-282, 1994; rotating metal elements, see Gianvittorio, J. P., Zendejas, J., Rahmat-Sami, Y., Judy, J., “Reconfigurable MEMS-enabled frequency selective surfaces”, Electronics Letters, vol. 38, issue 25, pp. 1627-1628, 2002; using a ferrite substrate, see Chang, T. K., Langley, R. J., Parker, E. A., “Frequency selective surfaces on biased ferrite substrates”, Electronics Letters, vol. 30, issue 15, pp. 1193-1194, 1994; pressurizing a fluid, see Bushbeck, M. D., Chan, C. H., “A tunable, switchable dielectric grating”, IEEE Microwave and Guided Wave Letters, vol. 3, issue 9, pp. 296-298, 1993; using a varactor tuned grid array that is a kind of quasi-optic oscillator, see Oak, A. C., Weikle, R. M. Jr., “A varactor tuned 16-element MESFET grid oscilator”, Antennas and Propagation Society International Symposium, 1995; using an electro-optic layer, see Rhoads' patent (U.S. Pat. No. 6,028,692); using transistors, see Rhoads' patent (U.S. Pat. No. 5,619,366); using ferroelectrics between an absorptive state and a transmissive state, see Whelan's patent (U.S. Pat. No. 5,600,325).
Although the above-mentioned methods are used to tune the FSS, these methods are not ideal for use with a tunable antenna. Many of the above methods are not practical for rapid tuning because they use moving metal parts, or pumping dielectric liquids. Some of them include switching between discrete states using transistors, which is less useful than a continuous tunable surface. Others include only on and off states, and cannot be tuned in frequency. Others require bulk ferrite, ferroelectric, or electrooptic materials, which can be lossy and expensive. None of the prior art achieves the capabilities of the presently disclosed Tunable FSS (TFSS) technology, even though a need exists for those capabilities.
The present TFFS is able to pass electromagnetic energy in a particular frequency band through the radome, and deflect or reflect electromagnetic energy in other frequency bands, as shown, for example, in FIG. 4. It can also be tuned to an off state where it is deflective or reflective, or an on state where it is absorptive over a broad range of frequencies. Also some regions of the surface can be tuned to different frequencies while other regions of the surface can be set to an opaque state, shown in FIG. 4.